Keeping baby awake while feeding?

Little background, my boy was born at 34 weeks 1 day and is turning 6 weeks old next week. He was in the nicu for 3 weeks and has been home 2 and a half. He is formula fed and is supposed to be fed every 3 hours, but we have found it better to feed him on demand. About half of the time, he won't stay awake for a full feeding, and sometimes can barely get through half of it before he falls asleep. Any tips on how to keep him awake through a feeding? The nicu said this was normal and to try and keep him awake as long as possible. Any tips would be appreciated 😁